25 kV AC
25 kV AC (kV = kilovolts, AC = alternating current, hence 25 000 volts alternating,) is one of the most common voltagess used for railway electrification, usually at 50 Hz or 60 Hz depending on that country's normal electricity supply frequency. It is always supplied via overhead lines.Overview
This voltage is ideal for long distance or heavy duty use, and for new suburban railways where brand new bridges and tunnels can be built to suit the larger structure gauge that this higher voltage system needs compared to 750VDC and 1500VDC.Railways using older and lower capacity direct current systems such as South Africa, Spain and Italy have or are introducing 25 kV AC instead of 3 kV DC for their new high speed lines.
History
25kV wasn't practical before the 1950s, and the development of high power and high voltage electronics such as thyristors. In the intervening years, those sorts of problems have been solved, while the equipment has been make more compact and reliable.
It is not clear why 25kV was chosen. 25kV is a quarter of 100kV. But electrical transmission lines are often 11kV, 33kV, 66kV and 132kV, so why introduce a voltage that does not fit the pattern?
On the other hand, one advantage of using 25kV for railway use, is that is clear that the railway power lines are being referred to, and not the ordinary 33kV or 66kV.
15kV 16.7Hz
The earliest high voltage electrification schemes circa 1900 were at 15kV in Germany, Switzerland, Sweden and Norwauy, but these used a lower frequency because of problems with commutator arcing at normal household frequencies such as 50Hz. Having to provide a separate grid at 16.7Hz was an extra bother and cost.
World standard
25kVAC has become pretty much the world standard for high speed and heavy duty electrification schemes for railways, since the 1950s. The use of normal household frequency power (50Hz or 60Hz) eliminated the need for costly separate 16.7Hz power grid.
Dual Voltage
25kV is never used directly for traction or for ancilleries, it is transformed and rectified down to a lower voltage such as 1500V DC. A dual voltage locomotive can bypass the 25kV stage and feeded the 1500V directly when operating on the DC system.
50 kV AC
Several heavy freight lines passing through isolated areas have doubled the voltage from 25 kV to 50 kV to eliminated the need to run electric transmission lines parallel to the railway. In the United States, the Black Mesa and Lake Powell line runs 125 km (78 miles) between Black Mesa, Arizona and Lake Powell for the purpose of transporting of coal between the Peabody Coal Company's Black Mesa Mine near Kayenta and the Navajo Generating Station at Page. It has no substations and gets its power directly from the generating station. In Canada, a 132 km (82 mile) 50 kV line was built in 1983 by BC Rail on its Tumbler Ridge sudivision in northern British Columbia, connecting to the Quintette and Wolverine mines near Tumbler Ridge. It was electrified because of its proximity to the massive W. A. C. Bennett Dam and had only one power substation, located where the main transmission line crossed the railway. It was abandoned in August 2000 after the Quintette mine closed. In South Africa another 861 km (540 mile) 50 kV line was built in the mid-1970's for the export of iron-ore, and operates between Sishen and Saldanha at a frequency of 50 hertz (Hz) instead of the 60 Hz used in North America. It has only two power substations, one at each end of the line. The use of 50 kV has been limited to heavy freight lines because of safety concerns about the high voltage and the fact that 25 kV has been found to be adequate for passenger trains travelling up to 350 km/h.6.25kV AC
A few lines in the UK had problems with insufficiant clearances for 25kV in tunnels and under low bridges, and these were electified at 6.25kV, this being a quarter of 25kV allowing for simple tap changes on the main transformers.
After many years, insulation technologies improved, and tunnels deepened, allowing the 6.25kV sections to be converted to the standard 25kV.